§ 11.3.5. Structural Requirements

Retaining walls must be designed according to Division I Section 5 of AASHTO Standard Specifications for Highway Bridges , latest edition. The following additional requirements apply, depending on type of wall.

A.

Design Life

Design must be based on a 100-year service life that, from a structural standpoint, is essentially maintenance-free.

B.

Cast in Place Concrete

Joints, including waterstops where applicable, must be provided according to ACI Manual of Concrete Practice Standard 224.3R Chapter 8.

C.

Conventional Segmental Gravity Walls (without mechanically stabilized backfill)

Internal stability of segmental gravity retaining walls without mechanically stabilized backfill (mortared or dry-stack rock, boulders or pre-cast concrete units) must be analyzed according to NCMA Design Manual for Segmental Retaining Walls , latest edition. The minimum factor of safety for internal shear capacity must be at least 1.5 if product-specific information is available; otherwise, it must be at least 4. External and overall, or global, stability shall be analyzed according to AASHTO Standard Specifications for Highway Bridges , latest edition.

D.

Tie-backs, Soil and Rock Nailing

Tie-back walls must be designed according to PTI, Recommendations for Prestressed Rock and Soil Anchors, 1996 edition or newer.

Soil and rock nail walls must be designed according to FHWA Manual for Design & Construction Monitoring of Soil Nail Walls and the University of Texas Center for Transportation Research Report 1407-1F, Rock Nail Design Guidelines for Roadway Cuts in Central Texas . Steel anchors must be corrosion-protected by epoxy coating or by encapsulation. Steel anchors protected only by grouting will not be permitted. In all cases, rock nails must be used in conjunction with shotcrete and a fascia wall.

Surface drainage must be prevented from infiltrating behind the wall or flowing over the wall by installing an interceptor ditch behind the top of the wall. To control groundwater seepage, composite geosynthetic face drains must be installed on the exposed rock face before shotcreting. The face drains must extend the full height of the wall and must connect to a base drain that discharges from behind the wall in a manner that water is not directed onto the adjacent sidewalk or into the street.